1. Field of the Invention
The present invention relates to an image forming apparatus such as a copier, printer, facsimile machine, or the like, and more particularly, to an image forming apparatus that transfers a toner image from a toner image carrying member to a recording medium.
2. Description of the Background Art
Typically, in image forming apparatuses, a toner image developed on a latent image carrying member is transferred to a recording medium such as transfer sheet either directly or via an intermediate transfer member. The toner image is then fixed on the recording medium by using a fixing unit. The recording medium is transported along a transportation route inside the image forming apparatus using a transportation member, which moves the recording medium.
Thus, the recording medium passes through a transfer position (or transfer nip), at which the toner image is transferred to the recording medium from an image carrying member or an intermediate transfer member. Then, the recording medium passes through a fixing position (or fixing nip) in the fixing unit, at which the toner image is fixed on the recording medium. Finally, the recording medium is discharged to a sheet discharge tray or the like.
In such image forming apparatuses, the recording medium may be transported along the transportation route from an upstream side to a downstream side of transport direction using a plurality of transportation members disposed at a plurality of positions along the transportation route. For example, such transportation members may be a pair of rollers (paired rollers), in which two rollers facing each other rotate to transport the recording medium. Accordingly, the recording medium is transported from one pair of rollers, disposed at the upstream side of sheet transport direction, to another pair of rollers, disposed at the downstream side of sheet transport direction. Such rollers may be referred to as surface moving members.
In such image forming apparatuses, one or more guide members may be disposed between adjacent transportation members, set along a transport direction of the recording medium. The guide member guides the recording medium from one transportation member, disposed at the upstream side of sheet transport direction, to another transportation member, disposed at the downstream side of sheet transport direction, by controlling a movement direction and/or orientation of the recording medium.
In such image forming apparatuses, the recording medium may be transported in two ways: in a horizontal transportation type, the recording medium is transported in a substantially horizontal direction; in a vertical transportation type, the recording medium is transported in a substantially vertical direction.
In the horizontal transportation type, when the recording medium is transported between adjacently disposed transportation members disposed along the transport direction of recording medium, the recording medium can be easily guided by a guide member, disposed between the adjacent transportation members and under the transportation route for the recording medium. Such easy transportation may be conducted because the guide member is disposed under the transportation route, in which the force of gravity can be used to enable the recording medium to be easily guided by the guide member.
By contrast, in the vertical transportation type, adjacent transportation members may be disposed in a vertical direction along the transport direction of recording medium, and a guide member is disposed at a lateral side of the transport direction, which means the guide member is also disposed in a vertical direction. Consequently, the force of gravity cannot be used to guide the recording medium and as a result the recording medium may not be guided by the guide member effectively.
JP-2006-234906-A and JP-3981859-B discuss an image forming apparatus using the vertical transportation type. In such image forming apparatus, a transfer position (or transfer nip) is set by an intermediate transfer belt (used as an intermediate transfer member) and a secondary transfer roller, and a registration roller is disposed under the transfer position and a fixing unit is disposed above the transfer position. In such image forming apparatus, the intermediate transfer belt and the secondary transfer roller, which transport the recording medium at the transfer position while sandwiching the recording medium, a fixing roller, which sets a fixing nip in the fixing unit, and the registration rollers are used as transportation members, wherein such belt and rollers apply transportation force to the recording medium.
Further, at the transfer position, an entry-side guide member is disposed at an upstream side transport direction, and an exit-side guide member is disposed at a downstream side of sheet transport direction. In such image forming apparatus, the recording medium is transported from the registration roller to the transfer position along the transportation route. Accordingly, relative positions of the registration roller and the entry-side guide member, and relative positions of the transfer position and the exit-side guide member may be set to a given value. Consequently, if such relative positions deviate from the set value, the recording medium may not be accurately transported in the transportation route.
For example, if the recording medium is not accurately transported from the registration roller to the transfer position in the transportation route, a leading edge of the recording medium may be askew with respect to the transport direction, and the recording medium enters the transfer position with the leading edge askew. If the recording medium enters to the transfer position as such, a toner image may be transferred on the recording medium while an orientation of toner image may be slanted from a horizontal or vertical direction of the recording medium, by which an image may not be accurately formed on the recording medium.
Further, the recording medium is transported from the transfer position to the fixing nip along the transportation route. Accordingly, relative positions of the transfer position and the exit-side guide member, and relative positions of the exit-side guide member and the fixing nip may be set to a given value. Accordingly, if such relative positions deviate from the set value, the recording medium may not be accurately transported in the transportation route. Such deterioration of transportation of the recording medium in the transportation route may cause flipping movement of recording medium, which may wrinkle the recording medium (e.g., sheet) when the recording medium passes the fixing unit.
Such inconvenience may be solved by setting one configuration for a secondary transfer unit including a secondary transfer roller, an entry-side guide member, and an exit-side guide member. The entry-side guide member is disposed before a secondary transfer nip, and the exit-side guide member is disposed after the secondary transfer nip to guide a sheet in the secondary transfer unit. The secondary transfer unit may be detachably mounted to the image forming apparatus. Such secondary transfer unit may have a casing to support the secondary transfer roller, the entry-side guide member, and the exit-side guide member by fixing such members to the casing. If such configuration is employed, relative positions of a guide member and the secondary transfer roller can be set precisely, by which relative positions of the entry-side guide member and the transfer position, and relative positions of the transfer position and the exit-side guide member, may be maintained at a preferred level.
However, the position of the secondary transfer unit installed in a housing of the image forming apparatus may vary from one manufactured image forming apparatus to the next. For example, the secondary transfer unit may be attached to an outer cover of image forming apparatus. When such assembly is employed, a position of the secondary transfer unit installed in the housing of image forming apparatus may vary due to assembly tolerances. Such assembly tolerances may include tolerance of the secondary transfer unit to the outer cover, tolerance of the outer cover to the housing of image forming apparatus, or the like.
Further, recently, nip pressure at a secondary transfer nip is set higher. Such high nip pressure may cause deformation of components of image forming apparatus. Using stronger components may prevent such deformation but also result in an increase in manufacturing cost.
Further, such deformation caused by the nip pressure may vary from one manufactured image forming apparatuses to the next depending on the components used, by which a position of the secondary transfer unit in the housing of image forming apparatus may fluctuate. Further, instability in the position of the secondary transfer unit in the image forming apparatus may occur for other reasons.
Once positional instability in the secondary transfer unit occurs in the image forming apparatus, a position of the secondary transfer unit relative to the fixing unit and the registration rollers also becomes unstable. If positional instability occurs between the secondary transfer unit and the registration rollers, precise relative positioning of the registration roller and the entry-side guide member at the transfer position may become unattainable, by which the recording medium may not be transported accurately from the registration roller to the transfer position in the transportation route. If such inaccurate transportation occurs in the transportation route, an image may not be accurately formed on the recording medium.
Further, if positional instability occurs between the secondary transfer unit and the fixing unit, precise relative positioning between the exit-side guide member at the transfer position and the fixing nip may also become unattainable, by which the recording medium may not be transported accurately from the transfer position to the fixing nip in the transportation route. If such inaccurate transportation occurs from the transfer position to the fixing nip in the transportation route, another failure such as sheet jamming may occur.
Such sheet jamming caused by inaccurate transportation may occur between the registration roller and the transfer position, or between the transfer position and the fixing position. Further, such sheet jamming caused by inaccurate transportation may occur at other parts of the transport route. For example, a guide member may be disposed between two transportation members in the transport route, in which one transportation member is disposed at a upstream side of sheet transportation direction of recording medium and other transportation member is disposed at a downstream side transport direction of recording medium. In such configuration, if positional instability occurs between the two transportation members, the above-described sheet jamming may occur.